1. Field of the Invention
The present invention relates to a process for recovering alcohol from admixtures with hydrocarbons.
2. Related Information
Since the Clean Air Act Amendments of 1990 refiners have searched for ways to introduce oxygen into gasoline to produce cleaner burning reformulated fuels. In addition to methyl tertiary butyl ether (MTBE), other suitable ethers for this purpose are tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE). The ethers are produced by the reaction of an alcohol and an olefin using liquid phase reactors, reactive distillation reactors and various combinations.
One highly successful system using concurrent reaction and separation of the reactants from the reaction products by fractional distillation (called catalytic distillation) has been practiced for some time. The process is variously described in U.S. Pat. No. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873 all commonly assigned herewith.
Briefly, in a catalytic distillation etherification the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure, and also having a distillation zone containing inert distillation structure. In a preferred embodiment in the etherification of isobutene and/or isoamylenes the olefin and an excess of alcohol, e.g. methanol are first fed to a straight pass reactor wherein most of the olefin is reacted to form the corresponding ether, methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME). One type of straight pass reactor is operated at a given pressure such that the reaction mixture is at the boiling point, thereby removing the exothermic heat of reaction by vaporization of the mixture. The described straight pass reactor and process are described more completely in U.S. Pat. No. 4,950,803 which is hereby incorporated by reference.
The effluent from the straight pass reactor is then fed to the distillation column reactor wherein the remainder of the iC.sub.4.sup.= or iC.sub.5.sup.= 's are converted to the ether and the methanol is separated from the ether, which is withdrawn as bottoms. The C.sub.4 or C.sub.5 olefin feed stream generally contains only about 10 to 60 percent olefin, the remainder being inerts which are removed in the overheads from the distillation column reactor.
As noted above, in the etherification of olefins with an alcohol, there is preferably an excess of the alcohol available in the reactor. This means that there is an excess of alcohol, e.g. methanol, in the reaction distillation zone of the distillation column reactor. In the distillation column reactor the methanol forms a minimum boiling azeotrope with the unreacted hydrocarbons. If the net methanol flow into the column is higher than the azeotrope, the methanol concentration will increase until alcohol leaves with the bottoms product.
The methanol feed rate is thus best controlled to produce the highest methanol concentration within the catalyst bed while preventing methanol leaving with the bottoms. This results in close to the azeotropic concentration. The methanol must be separated from the hydrocarbons so that the hydrocarbons can be used for gasoline blending and to conserve methanol. The separation is usually achieved by washing the hydrocarbon/methanol mixture with water. The methanol is preferentially absorbed in the water phase, and the water phase is subsequently fractionated to separate the methanol.
The azeotropic concentration of methanol with the unreacted C.sub.4 's in the MTBE is only about 4%. The separation is thus relatively easy. However, when the methanol is near to 12% as in the case of C.sub.5 's, the separation requires much more water and more theoretical stages in the standard countercurrent contacting mode used. The azeotropes of the unreacted hydrocarbons with other alcohols vary similarly.